When tube is bent about a bending surface, problems frequently arise in kinking of the tube or at least in stretching of the tube so that the circumferential dimensions of both the inner and outer surfaces of the tube are not true in the region of the bend. Thus, for example, when round tube is bent, its cross-section may be appreciably off-round in the region of the bend. Thus, the cross-section may be a flattened oval. To alleviate such problems, tube is frequently bent over a flexible mandrel conforming to the internal shape of the tube. Thus, if round tube is to be bent, the internal mandrel will have a circular cross-section and if square tube is to be bent, then the internal mandrel will have a square cross-section. Various suitable flexible mandrels are known, for example, such as that described in U.S. Pat. No. No. 3,455,142 issued Jul. 15, 1969 to Roberts.
Although the use of flexible mandrels is generally satisfactory in maintaining the form of the tube during bending, the flexibility of the mandrel presents problems during loading of the mandrel into the mouth of the tube especially where the process is automated and loading is fast.
In rotary draw bending machines, for bending tubes or pipes, such as those used in automotive exhaust systems, heat exchangers and aircraft hydraulic systems, a straight tube is loaded into the machine and the machine performs several operations to form the tube around a radius block. The machine may automatically relocate the tube and perform more bending operations to form the completed part. It is often desirable to have the machine automatically loaded with the straight tube to minimize the manual effort required and to provide consistency in terms of the location of the weld seam on the tube.
Previous automatic loading devices have means for selecting straight tubes from a bin or rack and moving the tube into a position where it is aligned with the tube gripping device on the bending machine. The loading device then moves the tube onto the gripping mechanism such that the bending machine grips the tube. Alternatively, the loading device simply holds the tube in the aligned position and the bending machine moves its gripping mechanism forward to grip the tube. The means for loading the tubes into the machine is generally a simple pick and place mechanism which moves the tube from one known position (the output chute of a tube rack) to another known position (the loading point for the bending machine). An alternative means for loading the tube is a programmable robot device which is programmed to select tubes from a bin and load them into the machine.
While systems such as these have been successfully implemented on many bending machines, the task is made more difficult on machines which utilize a flexible internal mandrel to support the inner wall of the tube during bending operations. This is due to the fact that means to align the flexible portion of the mandrel with the tube is required before the tube can be loaded. Previous systems to overcome this problem have involved retracting the mandrel assembly inside of a stationary rigid sleeve to support the mandrel, loading the tube into a collet and then advancing the mandrel back into the tube for bending, as described in UK Patent No. 2043504 to H. Benteler et al. While this system has been successful, it requires a relatively complex modification to the bending machine and it is not feasible when an internal collet system is employed. It also involves a relatively large motion of the mandrel assembly and, thus, requires significant operation time.